[ci skip] Fix double actions on PRs from main Paper repo

.github/workflows/build.yml

@@ -10,6 +10,8 @@ on: [push, pull_request]
 
 jobs:
     build:
+        # Only run on PRs if the source branch is on someone else's repo
+        if: ${{ github.event_name != 'pull_request' || github.repository != github.event.pull_request.head.repo.full_name }}
         runs-on: ubuntu-latest
         strategy:
             matrix: